In view of increased demand for four-wheel drive vehicles, a plethora of power transfer systems are currently being incorporated into vehicular driveline applications for transferring drive torque to the wheels. In many vehicles, a transfer case is interconnected between the primary and secondary drivelines which is equipped with a dog-type mode clutch that can be selectively engaged for rigidly coupling the secondary driveline to the primary driveline to establish a part-time four-wheel drive mode. In contrast, when the mode clutch is disengaged, drive torque is only delivered to the primary driveline for establishing a two-wheel drive mode.
A modern trend in four-wheel drive motor vehicles is to equip the transfer case with an electronically-controlled transfer clutch in place of the mode clutch. The transfer clutch is operable for automatically directing drive torque to the secondary wheels, without any input or action on the part of the vehicle operator, when traction is lost at the primary wheels for establishing an on-demand four-wheel drive mode. Typically, the transfer clutch includes a multi-plate clutch assembly that is installed between the primary and secondary output shafts of the transfer case and which is actuated by a power-operated actuator in response to control signals sent from a controller. The control signals are typically based on current operating characteristics of the vehicle (i.e., vehicle speed, interaxle speed difference, acceleration, steering angle, etc.) as detected by various sensors. Thus, such “on-demand” transfer cases can utilize adaptive control schemes for automatically controlling torque distribution during all types of driving and road conditions.
In many instances, the vehicle operator is also permitted to select between the two-wheel drive mode and the part-time four-wheel drive mode in addition to the on-demand four-wheel drive mode. Specifically, when the two-wheel drive mode is selected, the clutch assembly is released for delivering all drive torque to the primary output shaft. In contrast, when the part-time four-wheel drive mode is selected, the clutch assembly is fully engaged for effectively locking the secondary output shaft to the primary output shaft. In such applications, a mode signal is sent to the controller which is indicative of the particular drive mode selected by the vehicle operator. Typically, the mode signal is generated by a mode selector device which is manipulated by the vehicle operator.
Currently, a large number of on-demand transfer cases are equipped with an electrically-controlled clutch actuator that can regulate the amount of drive torque transferred to the secondary output shaft as a function of the value of an electrical control signal applied thereto. In some applications, the transfer clutch employs an electromagnetic clutch as the power-operated actuator. For example, U.S. Pat. No. 5,407,024 discloses a electromagnetic coil that is incrementally activated to control movement of a ball-ramp operator for applying a clutch engagement force on a multi-plate clutch assembly. Likewise, Japanese Laid-open Patent Application No. 62-18117 discloses a transfer clutch equipped with an electromagnetic actuator for directly controlling actuation of the multi-plate clutch pack assembly. As an alternative, U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having a transfer clutch equipped with an electric motor that controls rotation of a sector plate which, in turn, controls pivotal movement of a lever arm that is operable for applying a variable clutch engagement force on a multi-plate clutch assembly. Moreover, Japanese Laid-open Patent Application No. 63-66927 discloses a transfer clutch which uses an electric motor to rotate one cam plate of a ball-ramp operator for engaging a multi-plate clutch assembly. Finally, U.S. Pat. No. 4,895,236 discloses a transfer case equipped with a transfer clutch having an electric motor driving a reduction gearset for controlling movement of a ball screw operator which, in turn, applies the clutch engagement force to the clutch pack.
While many on-demand clutch control systems similar to those described above are currently used in four-wheel drive vehicles, a need exists to advance the technology and address recognized system limitations. For example, the size, weight and electrical power requirements of the electromagnetic coil or the electric motors needed to provide the described clutch engagement loads may make such system cost prohibitive in some four-wheel drive vehicle applications.